1. Field of the Invention
The present invention relates to a thermoplastic extrusion assembly which provides for the effective production of a substantially well blended and homogenized extrusion mixture utilized to form an extruded foam product that contains minimal imperfections, such as from air or gas pockets, in a manner which minimizes potentially hazardous pressure build ups at the mixing assembly and therefore minimizes a pressure drop after mixing which could result in a reduction of the overall through put rate and a production slow down. Moreover, the present invention produces the substantially homogenized extrusion mixture in a manner which does not require complex or costly modifications or alterations to existing foam extrusion manufacturing procedures, while nevertheless increasing the quality of the extruded foam product produced.
2. Description of the Related Art
The field of art associated with thermoplastic extrusion, and particularly thermoplastic foam extrusion is quite specialized, and indeed, is quite different from that typically associated with metal, rubber, or non-foamed plastic extrusion. Specifically, foam extrusion requires an initial step of melting pellets, usually made of a thermoplastic material, and a subsequent step of mixing the melted or melting thermoplastic with a foaming agent, such as a fluorocarbon (CFC, HCFC and HFC) or hydrocarbon (propane, butane, etc.), and possibly other agents, nucleating agents, fire retardants and/or coloring agents, in an isolated extrusion environment, so as to form an extrusion mixture.
Moreover, the most effective foam extrusion techniques completely contain the extrusion mixture during the melting and mixing stages, maintaining the mixture in a non-foamed, viscous form until passed through an extrusion die and exposed to external forces. Indeed, it is when the extrusion mixture exits the die of the foam extrusion assembly that it will foam (i.e. inflate and stiffen) into its ultimately usable form, such as films, planks, and large sheets from which meat trays, egg containers, small containers for butter and jelly, and the like, are formed. Accordingly, precision is imperative in order to ensure that an effective and complete mixing of the ingredients is achieved, thereby providing for a more precisely configured and homogeneous extrusion mixture and product, and further to ensure that the entire extrusion system is well contained until the extrusion mixture passes through the die, thereby avoiding premature foaming of the extrusion mixture.
In addition to the above concerns associated with the formation of a foam product is the need to maintain the extrusion mixture at a rather precise extrusion temperature, corresponding the polymer or substance being used as the basis for the extrusion mixture, so as to achieve a proper viscosity of the extrusion mixture and permit proper forming of the extrusion mixture through a die, such as a profile die, tube die, sheet die, annular die, flat die or several other types common types of dies. The rather precise extrusion temperature at which a desired range of viscosity is achieved is unfortunately, less than the initial "melt temperature", i.e. temperatures at which the pellets of extrusion material are melted, but cannot be too much less than the initial "melt temperature" for reasons about to be explained. As such, a substantial balance must be maintained. For example, if the melted extrusion mixture is permitted to cool too much, it will become too viscous and will fail to achieve the desired product density, becoming unusable, and will generally not effectively move through the extrusion assembly, let alone, out through the die. Conversely, if the temperature of the melted extrusion material is too high, its viscosity decreases significantly and the material is not dimensionally stable or shapeable as it flows through and from the die.
A further important consideration when forming the extrusion mixtures relates to the fact that the plastic material pellets utilized as a basis for the extrusion mixture, when melted, form a generally viscous, smooth liquid, which when infused with the liquid and/or gaseous foaming agent(s), or blends thereof, tends to maintain a plurality of fluid pockets dispersed therethrough, with a low viscosity resulting in the vicinity of the pockets, due to the plasticizing affect of the foaming agent, and a relatively high viscosity resulting in the regions lacking foaming agent. For this reason, it is necessary to achieve proper and complete mixing and blending of the foaming agent(s) with the liquid melted material pellets so as to generally minimize a size of the fluid bubbles contained in the extrusion mixture produced and provide a fuller, more homogeneously dispersed product for foaming. In particular, when the extrusion mixture exits the thermoplastic extrusion assembly, it is typically the foaming agent contained therein which upon being released from the pressurized environment tends to expand the extrusion mixture and causes the extrusion mixture to "foam" into the extruded foam product. Of course, if greater concentrations of the foaming agent are contained in certain portions or in certain pockets throughout the extrusion mixture, non-uniform foaming results, with certain portions foaming more than others, and imperfections in the structure of the extruded foam product, such as from air or gas pockets, are created. As such, the quality of the extruded foam product and the useable quantities of the extruded foam product actually produced are reduced. Furthermore, as the extruded foam product is often formed into large sheets, which are ultimately used for the finished product, imperfections in the sheets of extruded foam product can often significantly complicate or slow down the process of cutting or forming the finished product, as the imperfect or impure areas must be identified, if possible, before or after the formation of the finished product.
An additional concern associated with the foam extrusion process involves the fact that the melting and mixing generally takes place within an enclosed environment such that the addition of a foaming agent(s) tends to increase the pressure within the enclosed extrusion environment. Accordingly, a great degree of care must be undertaken in the mixing process in order to ensure that the steps which are undertaken to provide for substantially homogeneous mixing of the foaming agent(s) with the melted material pellets, does not also result in a potentially dangerous pressure build up at the mixing area of the enclosed environment. For example, such a pressure build up within the enclosed extrusion environment can lead to leaks, ruptures, cracks or other very dangerous breakdowns in the structure of the extrusion assembly after extend periods of use. Moreover, care must also be taken to avoid substantial pressure drops after the mixing area. Typically pressure drops will result from the abrupt release of an extended, restricted flow of the extrusion mixture, such as after an excessive pressure build up, and can significantly disrupt the smooth and effective movement of the extrusion mixture through the assembly and diminish the through put rate achieved so as to result in a drop off in the production rate.
Presently in the art, elongate generally dumbbell shaped structures 90, as illustrated in FIG. 2, are generally provided to ease foaming agent injection by temporarily discontinuing the presence of screw flights and diminishing pressure pulsation at the foaming agent injection hole in the extrusion cylinder, and for the mixing of the melted material pellets and foaming agent(s). In particular, these structures will typically include elongate gapped passages 92 between the structure 90 and the interior surface 36 of the barrel 35 so as to achieve some mixing and disbursement of materials, the gaped passages 92 connecting a single, large central trough 94 at which the foaming agent(s) is typically added to the melted material pellets. Unfortunately, however, such existing mixing structures are often substantially limited in the completeness to which they are able to mix the melted material pellets and the foaming agent(s). In particular, because of the size of the gaps, such existing structures 90 do not achieve effective homogenization, and even if the prior art recognized the benefit of reducing the diameter of the gaps, the gaped passages of the prior art could not be further reduced in size because as the extrusion mixture passes through the elongate gap region 92, a substantial pressure build up entering the structure 90 would result, and once the mixture leaves the structure 90 a substantial pressure drop would also result so that the mixture does not effectively flow and the overall through put rate suffers. Moreover, even with existing structures, the pressure build up can lead to potential ruptures or cracks in the barrel at the mixing point because of the increased pressure resulting from the extended, restricted urging of increased amounts of the extrusion mixture through the elongate gaped passages 92.
As a result, there is a substantial need in the art to provide an extrusion mixing assembly which is safe and effective to utilize, does not provide potentially harmful pressure variations, and does not result in pressure drops that would diminish the production rate of the system, but which also provides a substantially homogenized and substantially highly disbursed extrusion mixture which when passed through a die for forming will achieve a highly uniform and relatively imperfection-free extrusion product.